PterosomaLesson 1827

Pterosoma planumLesson 1827

Introduction

Pterosoma is a monotypic genus represented by the Indo-Pacific species P. planum. The transparent trunk, with whitish minute dish-shaped inclusions and surface tubercles, is expanded as an oblong disk that is flattened dorso-ventrally. The trunk terminates in a tapering tail. The visceral nucleus arises from a short, mobile stalk and is covered by a flattened shell. The shell is divided medially into two lateral halves by a low carina, homologous with the keel in Carinaria. The carina runs the length of the shell, beginning at the posterior end abutting the protoconch. While swimming, the visceral nucleus is raised dorsally, exposing the gills to inflowing water, but if disturbed it is retracted into a depression on the trunk surface. The proboscis is moderately short, extending ventrally beneath the anterior margin of the trunk.

Brief Diagnosis

Carinariid heteropods with:

Trunk expanded, forming an oblong disk that is flattened dorso-ventrally

Visceral nucleus arises from short stalk and is covered by a flattened shell

In young, post-metamorphic individuals, the expanded trunk disk (about 5 mm in length in the specimen below) is well developed and is fringed by about two dozen, prominent white tubercles. Numerous small white inclusions are also evident in the disk. Also, at this stage the visceral nucleus stalk is prominent and the adult shell (teleoconch) is just beginning to form (see second image below)

Larval shell globular. Right side with smooth surface except for low radiating ridges on second whorl (first image below). Spire low and aperture broadly oval and indented medially (second image below). Left side of shell with open umbilicus, having about 12 radiating striae on wall (third image below)

Behavior

Observations of free-swimming Pterosoma planum in aquaria aboard ship (Seapy, unpublished) revealed that the stalked visceral nucleus is normally elevated dorsally, exposing the gills and the visceral nucleus to inflowing water (see photographs under 1.a above). However, when individuals were prodded with a blunt probe, the visceral nucleus (covered dorsally by the flattened shell) was rapidly retracted into a longitudinal groove on the dorsal surface of the trunk. Hypothetically, the act of prodding an individual simulates disturbance by a predator, and the vulnerable soft tissues of the gills and visceral nucleus are protected beneath the shell.

In the book on holoplanktonic gastropod snails by Lalli and Gilmer (1989:32), the in-situ field observations of swimming Pterosoma planum by Gilmer were described: "Pterosoma has large opalescent spots covering the central body in both sexes; these spots continually change in brilliance and pattern as the animals swim." While the "central body" undoubtedly refers to the trunk, the nature of the "opalescent spots" is unclear since the only structures clearly visible on the trunk surface in net-captured specimens (Seapy, pers. obs.) are the surface tubercles described above under 1.b. Conceivably, the "opalescent spots" and the behavior observed by Gilmer may be limited to animals in-situ. Despite observing active and apparently healthy specimens swimming freely in aquaria aboard ship for extended periods of time (Seapy, pers. obs.), the observations of Gilmer were never seen.

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PterosomaLesson 1827. Pterosoma planumLesson 1827.
Authored by
Roger R. Seapy.
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